The Avalonian-Cadomian Belt and related peri-Gondwanan terranes

Abstract

During the past decade it has widely been accepted that the Avalonian-Cadomian belt results from Neoproterozoic to early Paleozoic Andean-type orogeny, that was active at the northern margin of Gondwana. The major imprints of this orogeny are recorded by so-called periGondwanan terranes which faced open oceans during the Neoproterozoic. Avalonianand Cadomian-type terranes show significant differences concerning the age of protoliths and the timing of accretion. Avalonian-type terranes developed from ca. 1 Ga old rocks and were subsequently accreted to the northern margin of West Gondwana (Amazonia). Cadomian-type terranes originated along the African margin by recycling ca. 2 Ga crust. In Paleozoic times the Avalonian-Cadomian belt fell into microplates that were detached from Gondwana and collided with Laurentia or Baltica. Several attempts have been made to reconstruct the Neoproterozoic-Early Paleozoic distribution of the peri-Gondwanan microplates relative to the West African and Amazonian Cratons. However, as robust paleomagnetic and other quantitative data are largely lacking, these reconstructions are still speculative and need further confirmation. The contributions of this special volume provide new data and models which help to improve our knowledge on the AvalonianCadomian orogeny. The papers are an outcome of the GEO 2002 meeting held during October 2002 in W rzburg, Germany. In an invited paper, Murphy et al. compare inferred relationships between peri-Gondwanan terranes and the northern Gondwanan margin with paleomagnetically constrained movements interpreted for the Amazonian and West African cratons for the interval ca. 800 – 500 Ma. The following eight papers deal with Cadomian events in the Bohemian Massif. A large number of new isotopic data is presented which helps to constrain the timing of orogenic events. The paper by Linnemann et al. represents new U-Pb SHRIMP ages of detrital and inherited zircons from pre-Variscan rocks of Saxo-Thuringia which reflect a West African provenance for sediments and magmatic rocks. Based on single zircon Pb-Pb evaporation and SHRIMP ages, combined with major and trace element data and Sm-Nd isotope systematics, Mingram et al., on the other hand, suggest a link between Saxo-Thuringia and Avalonia. They found evidence for at least two discrete magmatic events in the so-called red gneisses of the Erzgebirge. Patocka and torch elucidate Cadomian sediment provenances of the Early Paleozoic siliciclastic rocks of the Tepl -Barrandian unit, Bohemian Massif. The paper by Drost et al. presents new U-Pb-SHRIMP ages for the Cambrian volcanics of the Tepl -Barrandian unit and yields constraints on the late Neoproterozoic to Cambrian geotectonic settings. U-Pb and Pb-Pb ages obtained by Mazur et al. and by Zelazniewicz et al. indicate Neoproterozoic to Cambrian magmatism in different parts of the Sudetes (Bohemian Massif). Teipel et al. obtained new U-Pb SHRIMP and Nd isotopic data which suggest Upper Vendian and Lower Ordovician magmatism in the Bavarian Forest that is part of the Moldanubian unit. Friedl et al. derived new U-Pb zircon ages (SHRIMP and TIMS) from different orthogneisses of the Austrian part of the Bohemian Massif. They found evidence for both Neoproterozoic and Ordovician magmatism in the Moldanubian domain, the latter is interW. D rr Institut f r Geowissenschaften und Lithosph renforschung, Universit t Giessen, Senckenbergstr. 3, 35390 Giesen, Germany